1. Field of the Invention
The present invention relates to a lighting system and an image display apparatus using the lighting system.
2. Description of the Related Art
As one of the conventional image display apparatuses, there has so far been proposed an optical projector adapted to illuminate a liquid crystal panel which displays an image and project onto a screen a light reflected from or transmitted through the liquid crystal panel. Normally, such a projector uses a metal halide lamp, halogen lamp or xenon lamp as a light source. However, such lamps as a light source are disadvantageous in some respects which adversely affect the utility value of the lamps, as will be described below.
First, the lamp has a short service life. For example, the metal halide lamp has a light of about 2,000 hours. Therefore, the lamp has to be replaced frequently. For easy replacement when the service life has expired, the lamp has to be housed in a removable cartridge, for example.
Further, the projector is adapted to extract three primary colors (red, green and blue) from a white light from the lamp. So, the optical system for this RGB extraction is unavoidably designed to have a large volume, the color reproduction domain is limited and the efficiency for light utilization is lower.
To solve these problems, it has also been tried to use as a light source a semiconductor device such as light-emitting diode (LED) or a semiconductor laser. Generally, the LED has a service life of more than 10,000 hours, for example. However, the directivity of light from the LED is low. Namely, the light is emitted from the LED divergently. Concerning the LED, the efficiency for light utilization cannot easily be improved.
In this respect, it can be said that the semiconductor laser can emit light with an excellent directivity. Thus, the light from the semiconductor laser can be utilized with a high efficiency. Further, the semiconductor laser has a long service life. Generally, the semiconductor laser has a higher efficiency for energy utilization than the LED. Moreover, because of the monochromaticity, a large color reproduction domain can be set with the semiconductor laser.
When the semiconductor laser is used as a light source for the above-mentioned projector, however, its speckle noise is a problem in the application in consideration as will be described below.
Generally, it can be regarded that when a laser source is used as a light source in an image display apparatus, for example, various points and areas on an object surface or a screen, for example, which is thus illuminated are collectively contributed to formation of an image on an image surface or a viewer's retina, for example. In this case, it is natural that the object surface has a roughness which is larger than the magnitude of the wavelength of the light. Namely, light beams in a complicated phase relation with each other overlap on each other on he image surface. If the light beams are coherent with each other, the result of the coherence will cause a complex light-dark pattern. The complex light-dark pattern is a "speckle" which would considerably degrade the image quality in an image display apparatus, if applied. Generally, the semiconductor laser is sufficiently coherent to arise the speckle noise.
For another image display with a laser light, a laser scanning type image display apparatus has also been proposed. Also this laser scanning type image display apparatus unavoidably incurs the aforementioned speckle noise problem. Generally, the laser scanning type image display apparatus is basically configured such that an outgoing light from a laser source is focused by a lens and projected as a spot on a screen, and a polarizer disposed in the optical path is used to scan the focused spot two-dimensionally on the screen to display an image. Thus, the human eyes will see a light reflected from or transmitted through the screen.
In this case, on the image surface of the retina, the light beams within the focused spot will overlap on each other at an image point with a random phase change on the screen. Thus, the difference in optical path length between the light beams overlapping on each other at the image point will extremely be small, and so the light beams will cohere with each other, resulting in the speckles.
The speckle noise is a problem experienced commonly with the semiconductor lasers and highly coherent laser sources. Many approaches have ever been proposed to solve this speckle noise problem. A typical one of the conventional approaches is to use a rotary diffusion plate. That is, a random-diffusion plate made of a ground glass, for example, is inserted between an illuminating light source and a to-be-illuminated surface, the diffusion plate is rotated to vary, as the time passes, a speckle pattern occurring in an image surface, and the speckle pattern is averaged under a storage effect within the response speed of a light detection system. The response speed of the human eyes, for example, is said to about 30 msec. By rotating the diffusion plate at a sufficient speed for the speckle pattern to vary many rounds within the time of 30 msec, the speckle pattern can be made not recognizable by the human eyes.
Since the rotary diffusion plate has a nature to diverge the light, however, it will, when inserted in an optical system, cause a loss of incident light. Especially, if the rotary diffusion plate is used in the laser scanning type image display apparatus, it will cause a large loss of the light that can be focused on the screen. The rotary diffusion plate has to be driven to rotate by a motor, which will lead to a large volume of the optical system, a large energy consumption, an unignorable operating sound, etc. which all are not preferable for the image display apparatus as a household electric appliance.
Another approach has been proposed to reduce the speckle noise. It is to split a coherent light having some coherence length into a plurality of light beams having a difference in optical path length from each other, the difference being larger than the coherence length, and then join them together or array them. This approach is advantageous in that the light beams are incoherent with each other. Therefore, by splitting the coherent light into a larger number of light beams, it is possible to reduce the degree of spatial coherence with each other of the joined or arrayed light beams. As a well-known example of this approach, an optical fiber bundle has been proposed. In this approach, a plurality of optical fibers is bundled together. The optical fibers are cut to different lengths from each other for different optical path lengths from each other so that each of the differences in optical path length is set larger than the coherence length of an incident coherent light. The optical fibers are arranged to be flush at either end thereof with each other. Thus, when a coherent is incident upon one end of the fiber bundle, outgoing light beams from the other end of the bundle will be inherent with each other. Therefore, the spatial coherence is totally reduced. By using the outgoing coherent light beams as a light source in the image display apparatus, it is possible to reduce the speckle noise on the illuminated surface.
However, the aforementioned approach using the optical fiber bundle is disadvantageous as will be described below. It is assumed for example that fifty-one optical fibers having a difference in length of 1 cm from each other are bundled together for the above approach, the length difference between the shortest and longest optical fibers is 50 cm. For housing, in an image display apparatus, for example, the optical fibers bundled together to be flush at either end thereof with each other, the image display apparatus has to be designed large in volume, which will be a barrier against a compact design of the image display apparatus. Also, the numerical aperture at the incident end of the optical fiber bundle is smaller than 1, which will cause a loss of incident coherent light upon the fiber bundle. Further, light beams go out of the optical fibers at the other end, namely, light-outgoing end, of the optical fiber bundle, so that the outgoing light consists of divergent light beams going out of the light-outgoing ends each having an increased sectional area. This will cause a light loss at the downstream optical system. Furthermore, it is basically difficult to massively produce such fiber bundles. That is, this approach is also not suitable for employment in any household image display apparatuses.
Any means, if used, for providing the above-mentioned differences in optical path length between the optical fibers will not permit to sufficiently reduce the spatial coherence of a coherent light emitted from a coherent light source having a single-mode power spectrum since the coherence length of the coherent light is generally sufficiently long. In case a single-mode power spectrum semiconductor laser, for example, is used as a light source, the typical spectrum width is 100 MHz. Therefore, the coherence length will be 3 m or so. An optical system having such a large optical path length difference will have a considerably large volume, which will be a large barrier against employment of the optical system in the household image display apparatuses.